78 University of California Publications in Zoology [ Vou. 14 
MeGlashan and others (1912). The basis of computation has been 
somewhat different for the earlier and later sets of figures since 
different gaging stations were employed in the two cases. 
It will be seen here that the greatest annual discharge during the 
ten years considered is to the least as more than two to one. Vastly 
greater differences in the rate of river flow are however recorded for 
the different months of an average year. In table 21 it will be seen 
that the mean discharge for May is more than thirteen times that for 
September. On the other hand, the mean salinity of the bay as a 
whole for the April-May period of our observations was found to be 
such as to imply the presence of only two and one-half times as much 
fresh water as during the October period. Even allowing for the facet 
that during the year 1912 the river flow for April and May was less 
than half the normal, it will be seen that there can be no simple in- 
verse proportion between the salinity of the bay and the discharge 
of the rivers. Another phase of this same subject will be considered 
in the next few paragraphs. 
TABLE 21 
MeEAN MontHLY DISCHARGE OF SACRAMENTO AND SAN JOAQUIN RIVER SYSTEMS, 
Basep Upon REcoRDS FoR TEN YEARS 
Run-off in 
Month acre-feet 
October 527,356 
November 748,586 
December 1,352,997 
January 3,738,103 
February 3,453,295 
Mareh 4,710,754 
April 6,137,662 
May 6,467,948 
June 4,854,338 
July 2,027,965 
August 766,125 
September 490,364 
Total 35,275,493 
The mean annual discharge of the two river systems based upon 
records covering ten years has been shown to be about 35,600,000 aecre- 
feet of which about 26,100,000 comes from the Sacramento and its 
tributaries and about 9,500,00 from the San Joaquin and its 
tributaries. 
There are, in the course of a year, about 706 ebb tides, during 
which water passes from the bay into the ocean. The amount of 
